Statistical methods in finite element analysis.

Accelerated aging for testing polymeric biomaterials and medical devices

A spinal implant system for restricting flexion of a spine includes an elongate band proportioned to engage at least two spinous processes. During use, the band is positioned engaging the spinous processes at a spinal segment of interest, where it restricts flexion at the segment. The length and tension of the band may be adjustable following to implantation using percutaneous or transcutaneous means.

Spinal implant and method for restricting spinal flexion

A growing number of interspinous process devices have been introduced to the lumbar spine implant market Implant designs vary from static spacers to dynamized devices Furthermore, they are composed of a range of different materials including bone allograft, titanium, polyetheretherketone, and elastomeric compounds The common link between them is the mechanical goal of distracting the spinous processes to affect the intervertebral relationship In contrast, the purported clinical goals are more variable, ranging from treatment of degenerative spinal stenosis, discogenic low back pain, facet syndrome, disk herniations, and instability Though some clinical data exist for some of these devices, defining the indications for these minimally invasive procedures will be crucial Indications should emerge from thoughtful consideration of data from randomized controlled studies

Interspinous process devices in the lumbar spine.

The fundamental difference of the Ilizarov fixator is the type of pins used for bone fixation, i.e., Kirschner wires 1.5 mm and 1.8 mm in diameter, pretensioned from 50 to 130 kg before being affixed to the rings that are connected and fixed by threaded rods. The mechanical characteristics of external fixators may influence the biologic environment at the fracture site and ultimately decide the outcome of a surgical procedure. Thus, knowledge of the mechanical properties of the Ilizarov fixator is essential to a surgeon using it in clinical practice. The main objective of this study was to quantitate the mechanical behavior of the standard Ilizarov fixator under different loading conditions and fixator frame/wire configurations

Mechanical performance of Ilizarov circular external fixators in comparison with other external fixators : Newest knowledge of external skeletal fixation devices

Study design Mechanical testing of cadaveric lumbar spines and dual energy radiograph absorptiometry scanning were performed. Objectives To devise a technique to measure the strength of lumbar spinous processes and to determine the bone mineral density of the vertebrae used. Summary of background data The spinous process has been identified as the weakest part of the anatomy to which a flexible fixation device can be attached. It was unknown if the spinous processes could withstand the forces applied by the device. Methods A hook was fitted to the spinous process of 32 lumbar vertebrae. A custom-built rig was designed to secure a vertebra to a materials testing machine. A loop of cord was passed over a bar mounted on the crosshead of the machine and around the two bollards of the hook. As the crosshead was raised, a tension was applied to the cord. Each vertebra was tested to failure. The bone mineral density of each vertebra was then measured using dual energy radiograph absorptiometry. Results Failure of the specimens occurred by failure of the spinous process, pedicles, or vertebral body. The logarithm (base 10) of the load (N) at which failure occurred was 2.53 +/- 0.3, which corresponded to a mean failure load of 339 N. The bone mineral density of each vertebral body varied between 0.263 and 0.997 g/cm2. A significant linear correlation was found between bone strength and bone mineral density (P Conclusions Specimens with a bone mineral density in the range of 0.263-0.997 g/cm2 failed at a mean load of 339 N when the load was applied through the spinous process hook of a flexible fixation device.

Spinous process strength.

This article reviews the problems which are encountered in defining the mechanical properties of natural tissues, and in replacing them with synthetic materials in the human body. It describes how death, ageing, degeneration, pathology and individual variability influence the properties of natural tissues. Experimental problems arise from degradation and testing conditions; these are illustrated by the properties of the nucleus pulposus of the intervertebral disc. Replacement of natural tissues by graft materials and the products of tissue engineering is then described. Synthetic replacement materials should be biocompatible, i.e. they should not cause adverse reactions in the human body. However, polymers which hydrolyse in the body fluids may be useful for implants which are intended to have a limited life or for controlled release of drugs. Synthetic implant materials may attempt to mimic natural tissues but there may be a problem of attaching them to the surrounding tissue. Artificial ligaments provide an example of implants of this kind. Total hip replacement is used to illustrate the successful use of conventional engineering materials. Finally, safety issues are described; an implant material must be biocompatible, have the required mechanical strength, be sterile and should be incorporated into a device so that its performance in the living patient can be monitored.

Biomaterials: defining the mechanical properties of natural tissues and selection of replacement materials

A prototype flexible fixation system for the lumbar spine was subjected to tensile testing to failure and cyclic tensile testing in order to determine any regions of weakness. The system consisted of a spinous process hook and two laminar hooks made of stainless steel (316L). Each laminar hook was attached to the spinous process hook by a loop of polyester braid secured by a crimped metal sleeve. In five tensile tests, the system failed by irreversible deformation of the spinous process hook at 2.5 ° 0.3 kN (mean ° standard deviation). In three cyclic tests, in which the applied tension varied sinusoidally between 0.04 and 0.4 kN at a frequency of 5 Hz, failure occurred after less than 400000 loading cycles. This occurred as a result of fatigue crack initiation and propagation in the spinous process hook. A finite element model showed a stress concentration in the region where the crack occurred, which raised the applied stress above the tensile fatigue strength of this stainless steel. The spinou...

Design of spinous process hooks for flexible fixation of the lumbar spine.

This study describes a computational method for predicting the mechanical response of any configuration of the Ilizarov external fixation system. Mechanical testing of each of the individual components (ring, threaded rod, and wire) of the Ilizarov system was used to determine the stiffness of each component. Finite element (FE) analysis was then used to model each of the individual components. Each model was tuned to match the mechanical testing. A modular FE modelling system, using a master input file, was then developed where the tuned FE models of the individual components could be generated, positioned, and interconnected to replicate a range of fixator configurations. The results showed that the stiffness predications from the FE modelling of the fixator configurations were consistently 10 per cent higher than the stiffness values obtained from the mechanical testing. The FE modelling system can be used to predict the characteristic response of the fixator configurations and clearly shows th...

Finite element modelling of the Ilizarov external fixation system

The aim of this study was to test mechanically a new flexible fixation system for the lumbar spine. This device incorporates loops of polyester braid which are secured by a crimped titanium sleeve. In tensile tests, all loops failed, by slippage through the crimped sleeve, at 434 +/- 25 N (mean +/- standard deviation from five loops) for a single crimp and 415 +/- 15 N (from five loops) for two crimps. The intact system was then tested according to the ASTM standard. In a static test, all five specimens failed by slippage of the braid through the sleeve. Initial slippage occurred between 600 and 700 N, but the mean maximum load sustained was 1090 +/- 140 N. Dynamic tests were performed on ten constructs at a frequency of 5 Hz, under a range of loading conditions. The maximum load applied in any of the tests was 825 N. Two constructs did not complete the required 5 x 10(6) test cycles because of fracture of their spinous process hooks. However, other tests, under the same conditions, showed no signs of failure. Fracture occurred as a result of fretting damage from the recommended stainless steel roll pins.

K J Mathias

Papers

Spinous process strength.

Biomaterials: defining the mechanical properties of natural tissues and selection of replacement materials

Design of spinous process hooks for flexible fixation of the lumbar spine.

Finite element modelling of the Ilizarov external fixation system

Mechanical testing of a flexible fixation device for the lumbar spine.